Gas meter

ABSTRACT

A gas meter for determining a gas mixture consumption is revealed which determines sensor signal values proportional to a flow rate, this gas meter being calibrated as an energy measuring unit. The calibration is based on a basic gas mixture. During the measurement of the gas consumption, a measured energy consumption value is multiplied by a correction factor which takes account, at least approximately, of the calorific value of a supplied gas mixture, this calorific value being determined by an external unit. By this means, it is possible, using a simple and cost-efficient gas meter, to determine the effective supplied energy and to bill costs according to the supply.

TECHNICAL FIELD

[0001] The invention relates to a method for determining consumption ofa gas mixture and to a gas meter according to the preambles of patentclaims 1 and 9, respectively. The method and the gas meter are suitablein particular for use in the domestic and commercial sector and inparticular for determining the usage of natural gas.

PRIOR ART

[0002] Currently, gas bills, in particular in the domestic andcommercial sector, are based exclusively on the gas volume which hasbeen used. Therefore, gas meters which are based directly on measuringthe volume of the gas which has flowed through, in some casescompensating for measurement errors which arise through temperaturechanges, are primarily used.

[0003] The gas meter which is most frequently used is the so-calledbellows gas meter as described by U. Wernekinck, Gasmessung undGasabrechnung [Gas measurement and gas billing], Vulkan-Verl., 1996,20-31. The bellows gas meter has two measuring chambers which arealternately filled and emptied again by the gas flowing through. Whileone chamber is being filled, it displaces the gas into the other. Thefilling and emptying steps are counted and, multiplied by the volume ofthe measuring chamber, result in the overall volume of the gas which hasflown through. However, since the volume of the gas varies with changesin the ambient temperature and pressure, these measurements are subjectto errors. In summer, when the gas is warm and takes up a larger volume,the consumer would pay more for the same calorific value of the gas thanin winter. For this reason, modern bellows gas meters are provided withsimple mechanical or electrical devices for temperature compensation, inpractice, however, these are rarely used. However, pressure fluctuationsare not taken into account.

[0004] WO 99/06800 has disclosed a gas meter which determines avolumetric flow rate. For this purpose, in a gas pipe, a firstthermistor detects the cooling behaviour and a second thermistor detectsthe current temperature of the gas, and a flow rate of the gas moleculesis determined from these parameters. A cell in which the coolingbehaviour of stationary gas is detected is also arranged in the pipe.Consequently, a calibration value can be obtained at any desired timewhen the gas pipe is operating. This calibration value can then in turnbe used to determine the volumetric flow rate from the cooling behaviourof the first thermistor.

[0005] Despite all these compensatory measures, gas meters which arebased on volumetric measurements are always prone to errors and lead toan incorrect gas bill. Moreover, a charging principle which is based onvolumetric consumption is unfair to the consumer. This is because hisgas consumption is determined not by the volume, but rather by thequantity of gas, i.e. the consumed mass of gas, and by the quality ofthe gas, i.e. its calorific value. The denser and the more high-qualitythe gas, the less volume is required to achieve the same efficiency,whether in a heating system, a hot-water system or a cooking area.

[0006] Therefore, German patent application No. 199 08 664.8, which hasnot yet been published, describes a gas meter which determines the gasmass flow rate and therefore takes account of the density of the gas. Todo this, it is preferable to use an anemometer, as is known from F.Mayer et al., Single-Chip CMOS Anemometer, Proc. IEEE, InternationalElectron Devices Meeting (IEDM, 1997), 895-898. The disclosure of thesetwo documents forms part of the following description.

[0007] However, in the gas meters which have been described above,fluctuations in the quality of the gas are not taken into account. Thesefluctuations are considerable in particular in the gas of natural gasand arise primarily because the composition of the natural gas differsaccording to its source. However, in the supply of gas, gases fromdifferent sources are supplied in mixed form, and the mixing ratio mayvary considerably depending on demand.

[0008] It is true that the prior art has disclosed appliances which takeaccount of the calorific value of a gas and determine an energyconsumption. For example, WO 00/11465 has disclosed an energy-measuringappliance which on the one hand has a bellows gas meter for measuringthe volume and on the other hand has a device for determining thecalorific value of a gas, this calorimetric device being based on anacoustic measurement principle. U.S. Pat. No. 6,047,589 has alsodisclosed an energy-measuring appliance which determines flow volume andcalorific value of a gas; in this case, both measurements are based onthe acoustic effect. Therefore, both energy-measuring appliances arecalibrated for volume measurement, in each case carrying out acalculation using the calorific value which is currently measured onsite and the volume measured value to obtain the desired energy value.

[0009] These energy-measuring appliances are therefore relativelycomplicated, having to carry out both a volume measurement and adetermination of calorific value and, moreover, having to link the twomeasured values obtained. Appliances of this type are therefore tooexpensive for use as standard gas meters in the domestic and commercialsector.

SUMMARY OF THE INVENTION

[0010] Therefore, it is an object of the invention to provide a methodfor determining consumption of a gas mixture, and a gas meter, of thetype described in the introduction, which make it easy to measure gasusage which is dependent on calorific value and which are thereforesuitable for use in the domestic and commercial sector.

[0011] This object is achieved by a method and a gas meter having thefeatures of patent claims 1 and 9, respectively.

[0012] The method according to the invention is based on the recognitionthat, when measuring a flow rate, in particular a mass flow rate, ameasured value or sensor signal changes depending on the calorific valueof the gas. This dependency has a fixed relationship which is afirst-order proportional ratio. Consequently, it is possible tocalibrate the gas meter according to the invention directly as anenergy-measuring appliance.

[0013] Further corrections which take into account fluctuations in thecomposition of the gas mixture can be performed independently of themeasurement of the gas meter. The determination of the requiredcalorific value of an actually supplied gas mixture can be done by anexternal unit which is locally separate from the gas meter.

[0014] The advantage, therefore, is that it is not necessary for everygas meter to be equipped with a unit for determining the calorificvalue. A single external unit is sufficient to supply a plurality ofconsumers and therefore gas meters which are connected to the same gasmains with the required information about the calorific value of the gasmixture used.

[0015] In a preferred variant of the method according to the invention,this external unit transmits the information about the calorific valueto the gas meter, and the gas meter itself carries out a correction tothe measured energy consumption value on the basis of this information.

[0016] In another preferred variant of the method, the gas metertransmits the energy consumption value or an energy consumption valuewhich has been totalled up over a defined period of time to a centralcontrol unit, in which this value is corrected using the informationrelating to the calorific value of the gas mixture used which waspresent during this period of time.

[0017] Further advantageous embodiments will emerge from the dependentpatent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In the text which follows, the subject matter of the inventionwill be explained in more detail with reference to a preferred exemplaryembodiment, which is illustrated in the appended drawings, in which:

[0019]FIG. 1 shows an excerpt from a gas pipe having a gas meteraccording to the invention;

[0020]FIG. 2 shows a comparison of a deviation of monthly means forcalorific values of natural gas and corresponding measured-value changesin the gas meter according to the invention;

[0021]FIG. 3a shows measured value errors with respect to an effectiveenergy value of a gas for a volume measurement;

[0022]FIG. 3b for a mass flow measurement, and

[0023]FIG. 3c for an energy flow measurement according to the invention.

WAYS OF CARRYING OUT THE INVENTION

[0024]FIG. 1 shows a gas line which is provided with a gas meteraccording to the invention. The gas line comprises a principal pipe 1,which is connected to a gas mains pipeline which is outside the buildingand is not shown here. This principal pipe 1 has a pipe constriction 10of defined cross section or has other means, introduced into the mainconduit pipe 1, for achieving a well-defined pressure drop (pressuredropper). A gas flows through the gas line. The gas is generally a gasmixture, the composition of which varies. This is the case, for example,with natural gas, the three principal constituents of which, namelymethane, propane and ethane, have a different weighting according to theorigin of the gas. However, these three combustible principalconstituents also have different calorific values, so that the calorificvalue of the resulting gas mixture fluctuates accordingly.

[0025] There is a gas meter which has a measuring means 2 fordetermining a gas mass flow and an evaluation electronics (not shownhere). In a simple embodiment, the measuring means 2 is arrangeddirectly in the principal or main conduit pipe 1. However, in thepreferred embodiment illustrated here, a bypass pipe 11, which forms abypass to the pipe constriction 10, branches off from the principal pipe1. The measuring means 2 is arranged in this bypass pipe. The measuringmeans 2 is preferably an anemometer, preferably a CMOS anemometer with apolysilicon structure in sandwich form, as disclosed in the publicationsJ. Robadey et al., Two dimensional integrated gas flow sensors by CMOSIC technology, J. Micromech. Microeng. 5(1995) 243-250, in F. Mayer etal., Scaling of thermal CMOS gas flow microsensors: experiment andsimulation, Proc. IEEE Micro Electro Mechanical Systems, (IEEE, 1996),116-121, and in F. Mayer et al., Single-Chip CMOS Anemometer, Proc.IEEE, International Electron Devices Meeting (IEDM, 1997) 895-898, andas proposed for use as a gas meter in the unpublished German patentapplication No. 199 08 664.8 which was mentioned in the introduction.

[0026] The measuring means 2 has a heating element and in each case onetemperature sensor arranged upstream and downstream of the heatingelement, at identical distances, as seen in the direction of flow. A gaswhich is to be measured flows over the surface of the measuring means 2and is heated by the heating element. The temperature sensors are usedto measure the temperature or temperature difference of the gas upstreamand downstream of the heating element, as seen in the direction of flow,resulting in a sensor signal S in the form of a voltage signal U, whichis proportional to the temperature difference ΔT. The heat transfer rateis dependent on the number of molecules per unit volume and therefore onthe mass of gas. Moreover, however, it is also dependent on thecalorific value of the gas mixture, i.e. on the type or composition ofthe gas mixture.

[0027] According to the invention, the discovery that the sensor signalvaries as a function of the calorific value of a gas mixture is nowused. This takes place when the appliance is being calibrated as avolume-measuring appliance and more particularly when the appliance isbeing calibrated as a mass flow meter. This dependency is illustrated inFIG. 2. In this, CW signifies a deviation in per cent of mean or averagemonthly values from the mean or average yearly value for the calorificvalue of natural gas. As can be seen, the calorific value fluctuates byapproximately 2%. A change in the sensor signal value S which has beenobtained by means of the measuring means 2 described above for aconstant gas flow is also illustrated and is denoted by ΔS. It can beseen that the sensor signal value changes in the same direction as andeven almost proportionally to the calorific value. This relationshipdoes not apply to monthly average values only but, of course, alsoapplies to instantaneous values, i.e. for an arbitrarily small timescale.

[0028] According to the invention, therefore, the gas meter or the meansfor determining the mass flow can be calibrated or standardized as anenergy-measuring appliance or unit. To do this, the procedure is asfollows:

[0029] In a first step, a number of N sensor signal values S({dot over(V)}_(N) ₂ _(,n)) are determined as a function of a volume flow rate ormass flow rate for a calibration gas, this taking place under standardconditions, i.e. at a defined temperature (for example 20° C.) and adefined pressure (for example at 1 bar). As stated above, these sensorsignal values are proportional to a gas mass flow rate for the measuringmeans 2 employed. The sensor signal values S({dot over (V)}_(N) ₂ _(,n))are inverted and are stored in the evaluation electronics of the gasmeter in the form of a sensor calibration curve F_(n)(S({dot over(V)}_(N) ₂ _(,n))) as a flow rate depending on the sensor signal S.

[0030] The calibration gas used is preferably nitrogen N₂ or air. Thesensor calibration curve F_(n)(S({dot over (V)}_(N) ₂ _(,n))) ismultiplied, in a second step, by a signal conversion or correctionfactor ƒ_(N) ₂ _(-CH) and a calorific value factor H_(CH) for a basicgas mixture, designated by the index CH, and is in turn stored. Thesignal conversion factor is in this case a conversion factor which takesaccount of the difference in the sensitivity of the measuring means 2when using a base gas mixture instead of the calibration gas, in thiscase nitrogen. The calorific value factor H_(CH) takes account of thecalorific value of this base gas mixture, i.e. its calorimetric value orcalorific value per unit of the flow parameter, i.e. per standard volumeor per kg. The base gas mixture used is preferably an average gasmixture which is typical of the area where the gas meter is used.

[0031] The product obtained results in a power P as a function of thesensor signal S

P=P(S)=F _(n)(S({dot over (V)} _(N) ₂ _(,n)))·F _(N) ₂ _(-CH) ·H _(CH)

[0032] which indicates the instantaneous gas consumption as energy perunit time. Therefore, by integration or summing over a specific periodof time, it is possible to determine the energy consumption E of a basegas mixture:

E=∫P(S)·dt=ƒ _(N) ₂ _(-CH)·H_(n) ·∫F _(n)(S({dot over (V)} _(N) ₂_(,n)))·dt

[0033] The gas meter is therefore already calibrated as a power orenergy measuring unit based on the basic gas mixture. In its operation,fluctuations in time in the composition of the gas mixture consumed,i.e. deviations from the composition of the basic gas mixture are,namely, at least partially taken into account automatically by acorresponding fluctuation in the sensor signal S. For this purpose, itis not necessary to continually update the effective calorific value H,which fluctuates with time, or its deviation from the calorific valueH_(CH) of the basic gas mixture.

[0034] As may be seen from FIG. 2, the account taken of the fluctuationswith time in the gas composition, as achieved by the calibrationaccording to the invention, is indeed qualitatively correct but is notquantitatively perfect. A further improvement is achieved by using,instead of the calorific value H_(CH) of the basic gas mixture, acalorific value {overscore (H)}, which at least approximately takesaccount of the calorific value of the gas mixture effectively drawn. Thevalue {overscore (H)} is, for example, determined by an appropriateaveraging over an arbitrarily large interval. In order to determine theenergy effectively supplied, therefore, the energy consumption valuemeasured and calibrated in terms of the basic gas mixture is multipliedby a correction factor {overscore (H)}/H_(CH).

[0035] This calorific value {overscore (H)} is advantageously determinedin an external unit, either by calculation or experimentally. This unitdoes not have to be located at the premises of the correspondingconsumer, but rather it is possible to use a single unit for an entireconsumer network. This may be part of a central facility or control unitor may be in communication therewith. Known means can be used todetermine the calorific value. It is advantageous that it is alsopossible for accurate and expensive measuring means to be used for thispurpose, since only a single appliance is required. Therefore, thisexternal unit measures at all times or at set times the calorific valueof the gas mixture flowing through the consumer network and stores thisvalue.

[0036] In a variant of the invention, the external unit supplies the oreach gas meter of the consumer network with information about thecalorific value {overscore (H)} of the gas mixture supplied. This maytake place at predetermined time intervals or in the event of asignificant change in the gas mixture. In this variant, the gas meterhas calculation elements for correcting the measured energy consumptionvalue using the information relating to the calorific value. In thiscase, the information comprises the correction factor, the calorificvalue or a code which can be assigned to the correction factor. In apreferred variant, the gas meter totals up the measured energyconsumption value over a certain time span or interval i, for example aweek or a month, and multiplies this value with the correction factor{overscore (H)}(i)/H_(CH), which contains a calorific value {overscore(H)}(i) averaged over the i^(th) time interval. As a result, thefollowing equation is obtained for the effective energy consumption overm time intervals:$E = {f_{N_{2} - {CH}} \cdot {\sum\limits_{i = 1}^{m}\quad \left( {{\overset{\_}{H}(i)} \cdot {\int_{i}^{\quad}{{F_{n}\left( {S\left( {\overset{.}{V}}_{N_{2,n}}\quad \right)} \right)} \cdot {t}}}} \right)}}$

[0037] and if in addition the signal conversion factors are averaged$E = {\sum\limits_{i = 1}^{m}\left( {{{\overset{\_}{f}}_{N_{2} - {CH}}(i)} \cdot {\overset{\_}{H}(i)} \cdot {\int_{i}^{\quad}{{F_{n}\left( {S\left( {\overset{.}{V}}_{N_{2,n}}\quad \right)} \right)} \cdot {t}}}} \right)}$

[0038] In another variant of the method, the gas meter transmits themeasured energy consumption value to a central unit, which multipliesthe measured energy consumption value by the correction factor. If theexternal unit is not integrated in the central unit, it also transmitsinformation about the calorific value of the supplied gas mixture to thecentral unit. Preferably, the gas meter and/or the external unit totalup and/or integrate their measured values over a defined period of timeand transmit the integrated value to the central facility.

[0039] In all variants, the correction of the measured energyconsumption value can be carried out at any desired time, i.e. includingwhen the meter is being read.

[0040] By virtue of the direct calibration as an energy-measuringappliance, the method according to the invention and the gas meteraccording to the invention allow inexpensive and fair charging for thegas used. The more accurate measurement method can be seen from FIGS. 3ato 3 c. These figures show how great a deviation in a measured energyvalue from an effective energy value of a gas mixture is. FIG. 3a showsthe situation in which a gas meter is calibrated for a gas volume flowmeasurement. The figure illustrates the volumetric flow rate {dot over(V)} as a function of the energy E, in this case for a conventionalbellows measuring appliance without additional temperature compensation.If the corresponding gas energy is determined from the volumetric flowusing an appliance of this type, the error is up to ±18%. The principalcauses for the error are temperature fluctuations, which generallyamount to at most approximately ±10%, and pressure fluctuations of atmost approximately ±5%. FIG. 3b shows a measurement error which hasarisen from calibration based on the mass flow, for example using themeasuring means 2 described above. This figure illustrates the mass flowrate {dot over (M)} as a function of the energy E. The maximum error isapproximately ±4%, approximately 2% resulting from the measuringappliance and a further approximately 2% from the fluctuation over timeof the composition of the gas mixture or the calorific value. FIG. 3cshows the measurement error when using the measuring means 2 describedabove which is calibrated, in accordance with the invention, for theenergy flow. The figure illustrates the energy flow rate or power {dotover (E)} as a function of the energy E. As can be seen from thefigures, an appliance which is directly calibrated for energy flowmeasurement reproduces the reality best, because, in this case, themeasuring means corrects fluctuations with time in the composition ofthe gas mixture automatically in the correct sense or direction of theenergy flow rate.

List of Reference Symbols

[0041]1 Principal pipe

[0042]10 Pipe constriction

[0043]11 Bypass pipe

[0044]2 Measuring means

[0045] CW Deviation of monthly mean values

[0046] S Sensor signal value

[0047] ΔS Change in the sensor signal value S

1. A method for determining a gas mixture consumption by means of a gasmeter, which determines sensor signal values (S) proportional to a flowrate, characterized in that the gas meter is calibrated as an energymeasuring unit, its calibration being based on a basic gas mixture (CH),and in that the gas meter determines a power (P) or an energyconsumption value (E).
 2. The method as claimed in claim 1,characterized in that for the calibration of the gas meter, sensorsignal values (S_(n)) are determined as a function of the flow rate of acalibration gas (N₂) and are stored in the form of a sensor calibrationcurve (F_(n)(S_(n))) in the gas meter, the sensor calibration curvebeing multiplied by a signal conversion factor (f) and by a calorificvalue factor (H_(CH)) for the basic gas mixture (CH) and the productobtained providing a gas consumption in an energy unit.
 3. The method asclaimed in claim 1, characterized in that a measured energy consumptionvalue (E) is multiplied by a correction factor ({overscore (H)}/H_(CH)),which takes account, at least approximately, of the calorific value({overscore (H)}) of a supplied gas mixture.
 4. The method as claimed inclaim 3, characterized in that the calorific value ({overscore (H)}) ofthe supplied gas mixture is determined by an external unit.
 5. Themethod as claimed in claim 4, characterized in that the external unittransmits to the gas meter information about the calorific value({overscore (H)}) of the supplied gas mixture.
 6. The method as claimedin claim 4, characterized in that the gas meter transmits to a centralfacility the measured energy consumption value and the external unittransmits to a central facility information about the calorific value({overscore (H)}) of the supplied gas mixture.
 7. The method as claimedin claim 4, characterized in that the external unit is the centralfacility.
 8. The method as claimed in claim 3, characterized in that thecorrection factor ({overscore (H)}/H_(CH)) takes into account acalorific value ({overscore (H)}), determined over a certain timeinterval, of the supplied gas mixture.
 9. A gas meter for determining agas mixture consumption, the gas meter having a means (2) fordetermining a sensor signal (S) proportional to a flow rate,characterized in that the gas meter is calibrated as an energy measuringunit, its calibration being based on a basic gas mixture, and in thatthe gas meter determines a power (P) or an energy consumption value (E).10. The gas meter as claimed in claim 9, characterized in that the gasmeter has correction means in order to multiply a measured energyconsumption value by a correction factor ({overscore (H)}/H_(CH)), whichtakes account at least approximately of the calorific value ({overscore(H)}) of a supplied gas mixture.